Mobs will attack workbenches and torches if they can see them. The advantage of campfires is that mobs don't attack them, however this is moot in an enclosed farm (and you should be using fencing). For an island however you may not need a fence.

Workbenches are necessary for getting the overlapping radius's correct, but once that's done you can replace them. The advantage of workbenches is they allow you to build and repair fences.

Deathsquitos can fly over fences. Double high fences can help (I tend to do this anyway to stop the blob raid from eating crops), if you use bonfires (also spawn blocking) you might get lucky and they fly into one - killing themselves.

My usual plains set up is a cleared out fuling village. Bench in the middle, benches around the perimeter behind fuling bone fences (mobs won't attack their own fence, useful while building/extra insurance if something breaks through my fence), and a double high stakewall around that. Hang roof pieces off the fence to protect benches from weather, and cover the central bench with half height walls and a roof. Short of a raid nothing even tries to get in.